발간논문

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In this paper, optimum sintering condition of SiC_w/Al alloy composites by cold press-sintering method have been investigated in relation with density and hardness of the composites through a process variables such as pressure, sintering time, sintering temperature and sizing. A large amount of SiC whisker was observed to be surrounded grain boundary when Al poweder was coarsened. Green density of the composites was decreased with an increase of the volume fraction of SiC whiskers upto 15% and increased as compacting pressure increases 500㎫ to 700㎫. After sintering, both density and hardness of sintered composites was decreased with increasing SiC whisker volume fraction. It was found that the volume of increases with the volume fraction of SiC whiskers, whereas the volume of aluminum alloy matrix decrease. The relative density of the composites approved to increase with the repressure, sintering time and temperature. In particular, the hardness was significantly decreased in the temperature range of 100∼250℃ and steadily decreased above 250℃. The hardness of the composites with 15% SiC whisker was eight times greater than that of aluminum alloy matrix at 450℃.

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The effects of current density and the mode of applied current(DC and/or AC) on the resistance/spark sintering of TiAl powders were investigated to develope a new consolidation technique for this intermetallic compound. PREPed r-TiAI powders ranging 180∼210㎛ in size were sintered for short time of period(∼2mins) applying two different pressure by step by step during the flow of high density current of DC and/or AC. It was found that the mixed mode of DC+AC was more effective for the densification than DC mode because the former mode was believed to allow TiAl powders inside graphite dies to reach higher temperature due to the sparking at the neck area and uniform current distribution in the powder compact by AC effects. Compacts of high relative density($gt;99%) and uniform lamellar structure were obtained at the condition of DC(540A/㎠)+AC(50 A/㎠ and 5 ㎑).

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This paper deals with a systematical evaluation of the effect of joint preparation on LASER butt weld quality of structural steel for automobile application. Test results demonstrated that the sound weld could be obtained only by proper combination of welding conditions. Permitted joint gap tolerance and offset for a defect free weld in the autogenous LASER butt welding of 3.2㎜ thick structural steel is about 0.3㎜ and 0.3∼0.4㎜, respectively. However, the gap and offset should be maintained as small as possible. When the joint gap and offset, are greater than a critical amount, weld bead becomes irregular and forms a concave ditch on the weld surface with a poor penetration. In case of the offset values were greater than 0.5㎜, unmelted zone, which would act as a root crack in service. was also detected.

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In situ observation of antiphase domain boundary on the ordering process in CuAu thin film was made by transmission electron microscopy. At 320℃, in CuAuII stability range, antiphase domains are nucleated in CuAu I phase and grow along[100 or [010] directions. In CuAu II phase which involved a antiphase domain with two orientation variants, a large domain absorbed a small one in order to eliminate the lattice strain of interface. At 270℃, in CuAu I stabilty range, domain boundaries disappear with the reduction of domains. The diffuse character of the periodic antiphase boundaries is discussed in comparsion with a high resolution image showing the nucleation and growth of antiphase domain.

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Solution heat treatment and more complex thermomechanical treatments were performed on the recently developed two near-α Ti alloys, Ti-1100 and IMI 834. Equiaxed, bimodal and β-transformed lamellar α+βmicrostructures were provided with both alloys. This study was intended to obtain the optimum mechanical properties and to rank these alloys. To investigate the mechanical properties of these alloys, tensile tests were conducted at temperatures ranging from room temperature to 700℃. High cycle fatigue(HCF) tests were performed at room temperature and at 400℃. Creep tests were done at 600℃. Test results exhibited that all the mechanical properties of the IMI 834 were slightly superior to those of the Ti-1100. Creep strength of the bimodal microstructure was always about 50MPa lower than that of the lamellar structure up to 700℃. Considering overall properties, however, the bimodal microstructure produced by a thermomechanical treatment done just below the Q-transus temperatures exhibited the most optimum properties. The thermomechanical treatments change the microstructures and consequently influence the mechanical properties. This process can be used to meet the special requirements for either room-or high-temperature applications.

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In determining limits to ductility of highly-stressed plastic materials such as aeroengine components, one of major importance is the establishment of a criterion for local microcavity initiation condition in terms of evolving stress and plastic strain. In this paper, critical conditions leading to microcavity initiation by local interfacial debonding between elastic carbide particles and the plastic matrix during straining were determined for two cases by considering the states of stress and strain, namely uniaxial tension and uniaxial tension with elevated triaxiality. The continuum analysis was first employed to calculate the critical interfacial strength, σ_c, in order to permit comparison to earlier calculations. Secondly, σ_c was estimated by incorporating local microstructural variables into a microscopic dislocation model, based on a critical stress criterion. The progress of microcavity initiation was also described with local interfacial stress, assuming that the microcavity initiation rate has a normal distribution about a mean interfacial stress, σ_M.

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Al₂O₃ fibers were coated with metallic cobalt by sol-gel method for the purpose of enhancing the wettability of fiber in molten metal and improving the interface properties of metal matrix composite. The Al₂O₃ short fibers were suspended in a cobalt sol containing cobalt acetate, then dried and reduced in hydrogen atmosphere so as to produce cobalt metal coating on the fibers. The results of X-ray diffraction confirmed that the coating layers were completely reduced to metallic cobalt at 300℃.

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